1The Compensation Formulas Behind This Tool
| Primary Disorder | Compensation Formula | Tolerance |
|---|---|---|
| Metabolic acidosis | Expected PaCO2 = 1.5 × HCO3 + 8 (Winter's formula) | ± 2 mmHg |
| Metabolic alkalosis | Expected PaCO2 = 40 + 0.7 × (HCO3 − 24) | ± 5 mmHg |
| Respiratory acidosis (acute) | Expected HCO3 = 24 + 1 × [(PaCO2−40)/10] | ≈ ± 3 mEq/L |
| Respiratory acidosis (chronic) | Expected HCO3 = 24 + 4 × [(PaCO2−40)/10] | ≈ ± 3 mEq/L |
| Respiratory alkalosis (acute) | Expected HCO3 = 24 − 2 × [(40−PaCO2)/10] | ≈ ± 3 mEq/L |
| Respiratory alkalosis (chronic) | Expected HCO3 = 24 − 5 × [(40−PaCO2)/10] | ≈ ± 3 mEq/L |
Why "Normal pH" Can Still Mean a Disorder
A pH of 7.38 with a PaCO2 of 65 and HCO3 of 36 looks reassuring at first glance — the pH is inside the normal 7.35–7.45 range. But both PaCO2 and HCO3 are markedly abnormal in opposite directions, which is the signature of a fully renal-compensated chronic respiratory acidosis (classic COPD picture), not a healthy ABG.
This tool checks PaCO2 and HCO3 against their own normal ranges independently of pH, then uses whether pH sits above or below 7.40 to work out which parameter is primary and which is compensating — the same logic taught in the step-by-step framework on the Anion Gap article.
Same-Direction vs. Opposite-Direction Abnormalities
When PaCO2 and HCO3 are abnormal in the same direction (both suggesting acidosis, or both suggesting alkalosis), that's not compensation — it's two primary disorders stacking on top of each other, and the pH is usually markedly abnormal as a result (e.g. cardiac arrest: high PaCO2 + low HCO3 together).
When they're abnormal in opposite directions, one is very likely compensating for the other — the tool then checks whether the compensating value falls inside its expected range, or whether it suggests a second, hidden disorder.
2Common ABG Patterns
| Clinical Picture | Typical ABG | Pattern |
|---|---|---|
| DKA | pH↓ PaCO2↓ HCO3↓↓ | Metabolic acidosis, HAGMA, compensated |
| Diarrhoea | pH↓ PaCO2↓ HCO3↓ | Metabolic acidosis, normal AG (NAGMA) |
| COPD exacerbation | pH↓/normal PaCO2↑↑ HCO3↑ | Respiratory acidosis, acute-on-chronic |
| Opioid overdose | pH↓ PaCO2↑↑ HCO3 normal | Acute respiratory acidosis |
| Vomiting / NG suction | pH↑ PaCO2↑ HCO3↓↓ | Metabolic alkalosis, compensated |
| Panic attack / anxiety | pH↑ PaCO2↓↓ HCO3 normal | Acute respiratory alkalosis |
| Salicylate toxicity | pH normal/↑ PaCO2↓↓ HCO3↓ | Mixed respiratory alkalosis + HAGMA |
| Sepsis with lactic acidosis | pH↓ PaCO2↓ HCO3↓ | Metabolic acidosis (HAGMA), often with hyperventilation |
3Frequently Asked Questions
Identify the primary disorder from PaCO2 and HCO3 independently, use pH to decide which one is primary when they move in opposite directions, and check the appropriate compensation formula before concluding a disorder is "simple." Same-direction PaCO2/HCO3 abnormalities mean a mixed disorder, not compensation.